This invention relates to semiconductor devices. More particularly, it is concerned with junction field effect transistors of the static induction type and to methods of fabrication.
The static induction transistor is a field effect semiconductor device capable of operation at relatively high frequency and power. The transistors are characterized by a short, high resistivity semiconductor channel region which may be controllably depleted of carriers. The current-voltage characteristics of the static induction transistor are generally similar to those of a vacuum tube triode. These devices are described by Nishizawa et al in U.S. Pat. No. 3,828,230 issued Aug. 6, 1974, and in U.S. Pat. No. 4,199,771 issued April 22, 1980.
The static induction transistor generally uses vertical geometry with source and drain electrodes placed on opposite sides of a thin, high resistivity layer of one conductivity type. Gate regions of opposite conductivity type are positioned in the high resistivity layer on opposite sides of the source. During operation a reverse bias is applied between the gate region and the remainder of the high resistivity layer causing a depletion region to extend into the channel region below the source. As the magnitude of the reverse bias is varied, the source-drain current and voltage derived from an attached energy source will also vary.
Recessed gate static induction transistors which have improved characteristics are described in United Kingdom patent application No. 2,026,237 which was published on Jan. 30, 1980, and in an article "High Performance Microwave Static Induction Transistors" by Cogan et al published in the proceedings of the International Electron Devices Meeting (IEEE) Dec. 5, 6, and 7, 1983, Washington, D.C., Paper 9.5, pages 221-224.
The design and fabrication of static induction transistors particularly those having recessed gate structures are difficult because of extremely tight dimensional tolerances. Alignment and registration of the masks employed in delineating the various regions of the device are critical. In addition the structural elements of the completed device such as the bonding pads may contribute to degrading the operating characteristics particularly at extremely high operating frequencies.